In optical telecommunication systems, modulated optical wave energy typically is transmitted from an optical source to a remote optical receiver through the medium of an optical fiber. The fiber supplies desirable waveguiding of the optical energy along a desired transmission path from source to receiver. A typical and useful source of the transmitted optical energy is a laser, such as a semiconductor diode laser having a double heterojunction structure ("double heterostructure"). For signaling purposes--i.e., in order to transmit information from source to receiver--the optical wave is modulated either in power or in frequency as a function of time. Either analog or digital modulation is used, depending upon a variety of technological and economic considerations. A typical and useful form of modulation is binary digital power modulation or pulse code modulation, that is, modulation where the optical power during each allotted time slot (or window) is made to be HIGH or LOW (ON or OFF) representing, respectively, a binary digital 1 or 0. Thus, each bit of the resulting data stream is transmitted as the presence vs. absence of optical power during each allotted time slot.
Known optical fibers, which have the lowest loss--and hence which can transmit digital data over relatively long distances (of the order of 100 km or more) without the need for signal regeneration ("repeaters")--have relatively high dispersion; that is, the fiber transmits different optical frequencies at different velocities. As a result of this dispersion, any relatively short pulse that contains more than one frequency (or more than one very narrow frequency band) is distorted during propagation, and hence the ability to detect the pulse is compromised, whereby an error at the receiver in the detection of the pulses tends to occur. Similarly, because of dispersion, any pulse containing a single substantially pure frequency (or very narrow band frequency spectrum) that is even only slightly different from the frequency of preceding pulses will arrive at the receiver during a time slot which is slightly different from the slot that has been assigned for that pulse, such assigning of the time slot having been based upon the supposition of precisely equal frequency for all pulses; thereby a similar error of detection occurs at the receiver. In either event, dispersion thus produces errors in the detection of the pulses.
A typical and useful form of laser source, therefore, is a modulated semiconductor heterojunction diode laser operating in a single electromagnetic mode which unambiguously produces a single substantially pure frequency output. Such single-mode operation is desirable to avoid undesirable confusion of the information carried by the wave, such confusion otherwise being caused by the above-mentioned dispersion, i.e., different frequencies would travel through the fiber at different velocities.
In U.S. Pat. No. 3,748,597, filed by H. K. Reinhart on July 24, 1973 entitled "Optical Modulator," a technique for modulating a semiconductor heterojunction diode laser section is described in which one end facet of the laser is spaced from an end facet of a semiconductor diode modulator section by a spacing layer composed of a partially transmissive and electrically insulating material. However, because of the disparate heterojunction structures of the cross sections of the laser and modulator sections and because of the thickness of the spacing layer, undesirable optical loss occurs in the transfer of optical radiation from one section to the other due to poor optical coupling between these sections.
Recently, W. Tsang et al, in a paper entitled "High-speed Direct Single-frequency Modulation with Large Tuning Rate and Frequency Excursion in Cleaved-coupled-cavity Semiconductor Lasers," published in Applied Physics Letters, Vol. 42, pp. 650-652 (April 1983), and in a pending patent application by W. Tsang, Ser. No. 482,964, filed Apr. 8, 1983, entitled "Multicavity Optical Device," have described a cleaved-coupled cavity (C.sup.3) laser source typically of optical wavelength about 1.5 micron. This source was formed by a pair of spaced, optically coupled, mutually electrically isolated, similar heterojunction diodes. The spaced relationship between the diodes was obtained by cleaving a monolithic diode structure located on an electroplated or evaporated gold pad to form a pair of physically separate diodes, followed by using the gold pad as a hinge to adjust the distance of separation between the diodes. Such a laser source has an output optical beam which can be adjusted in frequency by suitable adjustment of electrical currents delivered to the diodes.
Typically, for data transmission purposes, to one of the diodes is applied a steady (dc) current bias and to the other of the diodes is applied a sequence of mutually equal voltage pulses superimposed upon another steady current bias, this sequence corresponding to the sequence of digital 1's and 0's desired to be transmitted. Such a laser source has the advantage of single longitudinal mode performance. That is to say, once laser oscillation therein commences, so that a frequency of laser oscillation in a longitudinal mode is established in response to a voltage pulse applied to one of the diodes, the frequency (and hence wavelength of output beam) is pure and stable, i.e., is essentially a single frequency that remains the same for the entire duration of the pulse. Therefore, dispersion by the optical fiber located between source and receiver does not cause undesirable distortion of the optical pulses as they are being transmitted through the fiber. Accordingly, this laser may be used with a fiber optimized for minimum loss.
On the other hand, the mode and hence laser frequency of oscillation which is established may not be precisely the same for different pulses unless special precautions are taken. In other words, the mode of laser oscillation established in response to the current pulses may shift or jump after generation of a large number (say about 10.sup.9 or more) of such pulses. As a result, an undesirable event may occur when the output energy of such a laser source is transmitted to an optical detector through an optical fiber which suffers from dispersion: optical pulses corresponding to successive current pulses undesirably may not arrive at the receiver within their preassigned time slots or may even be reversed in times of arrival--that is, the optical pulse corresponding to a given current pulse may arrive at the receiver after (instead of before) the optical pulse corresponding to a subsequent current pulse. Such an undesirable event would constitute an error in signal transmission. Although the probability of such an error is quite small, typically of the order of less than one in a billion, it may be large enough to be of serious concern in binary digital data transmission systems where even such small error rates are not within desired system reliability specifications. Given the dispersion properties of present-day fibers, undesirably closer spacing between successive optical regenerators (or "repeaters") along the fibers to reconstitute the sequence of optical pulses would be required to prevent this kind of error unless special precautions are taken to avoid the shift in mode. Alternatively, the maximum useful pulse repetition rate would be undesirably limited: the higher the pulse repetition rate, the shorter the required pulse and hence the more easily slight changes in frequency would produce errors of arrival (during the wrong assigned time interval). Thus, although the laser described by Tsang et al is well suited for single mode fiber optical communication systems, unless precautions are taken, mode shifting may occur and cause errors in transmission.
Moreover, in the case of an internally modulated laser, as in the aforementioned Tsang et al paper, because of thermal effects produced by a succession of optical pulses, the amplitude (and hence power level) of a given pulse is signal pattern dependent, i.e., depends upon the actual signals (1's vs. 0's) during the preceding several time slots. Accordingly, the output power level of different pulses is variable, and in particular is undesirably degraded by a sequence of successive digital "1" signals. This degradation of power level necessitates an undesirable decrease in separation between regenerators along the transmission path, in order to accommodate the worst case signal sequence, i.e., a sequence of many successive 1's (typically as many as 15 successive 1's in present-day formats).
Accordingly, it would be desirable to have a laser source which, in response to a current pulse, operates in a single mode at a single frequency which is more reliably reproducible, i.e., which is more reliably of the same mode and of the same amplitude for successive current pulses than prior art.